Neoproterozoic Snowball Earth Research Articles

Sea ice-free conditions during the Sturtian glaciation (early Cryogenian), South Australia

In the Neoproterozoic snowball Earth hypothesis, shutdown of the planet9s hydrological system has been attributed to a global ice cover during one or more extreme glaciations. In the central Flinders Ranges, South Australia, the Yudnamutana Subgroup of Sturtian age includes diamictite, sandstone, and siltstone units of glaciomarine origin as much as 5000 m thick, and is overlain by postglacial transgressive siltstone and shale of the Tindelpina Shale Member, Tapley Hill Formation. In the central Flinders Ranges, the Yudnamutana Subgroup consists of (1) the Pualco Tillite (gravity resedimented glacial deposits), (2) the Holowilena Ironstone (glacioturbidites), (3) poorly stratified pebbly diamictite of the Warcowie Dolomite Member, lowermost Wilyerpa Formation (gravity resedimented glacial deposits), succeeded by (4) siltstones and sandstones with abundant hummocky cross-stratification (HCS: storm deposits), and (5) a lonestone-bearing succession with cobble-sized clasts in the upper Wilyerpa Formation (ice-rafted debris interpreted to record a glacial re-advance) in which HCS is absent. Because the action of oscillating waves is required to produce HCS on the seafloor, its presence indicates an interval of significant meltback prior to glacial re-advance. Given that the HCS occurs ∼2 km beneath the Tindelpina Shale Member, it signifies a major ice-free interval during the Sturtian glaciation.

Mudball: Surface dust and Snowball Earth deglaciation

Recent modeling results have raised doubts about the ability to deglaciate from a global glaciation at atmospheric carbon dioxide levels that are realistic for a Neoproterozoic Snowball Earth. Here we argue that over the lifetime of a Snowball event, ice dynamics should lead to the development of a layer of continental and volcanic dust at the ice surface in the tropics that would significantly lower the tropical surface albedo and encourage deglaciation. This idea leads to the prediction that clay drapes found on top of Neoproterozoic glaciations should be thicker in tropical than extratropical regions. We test this idea by running the FOAM general circulation model (GCM) with an added tropical dust layer of different sizes and albedos and find that the tropical dust layer causes Snowball deglaciation at pCO2 = 0.01–0.1 bar in a reasonable regime of these parameters. We find similar, though more nuanced, results from a limited number of test cases using National Center for Atmospheric Research's CAM GCM.

Toward the snowball earth deglaciation…

The current state of knowledge suggests that the Neoproterozoic snowball Earth is far from deglaciation even at 0.2 bars of CO2. Since understanding the termination of the fully ice-covered state is essential to sustain, or not, the snowball Earth theory, we used an Atmospheric General Climate Model (AGCM) to explore some key factors which could induce deglaciation. After testing the models’ sensitivity to their parameterizations of clouds, CO2 and snow, we investigated the warming effect caused by a dusty surface, associated with ash release during a mega-volcanic eruption. We found that the snow aging process, its dirtiness and the ash deposition on the snow-free ice are key factors for deglaciation. Our modelling study suggests that, under a CO2 enriched atmosphere, a dusty snowball Earth could reach the deglaciation threshold.

Megaphylogeny, Cell Body Plans, Adaptive Zones: Causes and Timing of Eukaryote Basal Radiations1

I discuss eukaryote megaphylogeny and the timing of major innovations in the light of multigene trees and the rarity of marine/freshwater evolutionary transitions. The first eukaryotes were aerobic phagotrophs, probably substratum-associated heterotrophic amoeboflagellates. The primary eukaryote bifurcation generated unikonts (ancestrally probably unicentriolar, with a conical microtubular [MT] cytoskeleton) and bikonts (ciliary transformation from anterior cilium to ancestrally gliding posterior cilium; cytoskeleton of ventral MT bands). Unikonts diverged into Amoebozoa with anterior cilia, lost when lobosan broad pseudopods evolved for locomotion, and Choanozoa with posterior cilium and filose pseudopods that became unbranched tentacles/microvilli in holozoa and eventually the choanoflagellate/choanocyte collar. Of choanozoan ancestry, animals evolved epithelia, fibroblasts, eggs, and sperm. Fungi and Ichthyosporea evolved walls. Bikonts, ancestrally with ventral grooves, include three adaptively divergent megagroups: Rhizaria (Retaria and Cercozoa, ancestrally reticulofilose soft-surfaced gliding amoeboflagellates), and the originally planktonic Excavata, and the corticates (Plantae and chromalveolates) that suppressed pseudopodia. Excavata evolved cilia-generated feeding currents for grooval ingestion; corticates evolved cortical alveoli and ciliary hairs. Symbiogenetic origin and transfers of chloroplasts stimulated an explosive radiation of corticates--hard to resolve on multigene trees--and opisthokonts, and ensuing Cambrian explosions of animals and protists. Plantae lost phagotrophy and multiply evolved walls and macroalgae. Apusozoa, with dorsal pellicle and ventral pseudopods, are probably the most divergent bikonts or related to opisthokonts. Eukaryotes probably originated 800-850 My ago. Amoebozoa, Apusozoa, Loukozoa, and Metamonada may be the only extant eukaryote phyla pre-dating Neoproterozoic snowball earth. New subphyla are established for Choanozoa and Loukozoa; Amoebozoa are divided into three revised subphyla, with Variosea transferred into Conosa.

Glacial Sedimentary Processes and Products

Research in the field of glacial sediments and sedimentary processes increasingly involves a range of different approaches including investigations of modern glacial processes, the paleoglacial record, and numerical ice sheet modeling. This diversity of approaches is reflected in this International Association of Sedimentologists special publication edited by Michael Hambrey et al., which stems from the 2005 “Conference on Glacial Sedimentary Processes and Products,” held at the University of Wales, Aberystwyth. Following a foreword about geologist Brian Harland (to whom the volume is dedicated) and the Neoproterozoic Snowball Earth theory, and an introduction by the editors, the book has four parts.Part 1, “Glacier dynamics and sedimentation,” focuses on modern glacier and ice sheet processes. It includes three very diverse papers. A paper by Siegert et al. explores the possibility of hydrological links between Antarctic subglacial lakes and the consequences of rapid drainage events for subglacial erosion and deposition. This is followed by a paper (Midgley et al.) on moraine geomorphology and sedimentology in Svalbard, Norway, and a final paper (Rousselot et al.) on the description of a laboratory apparatus for investigating clast ploughing in sediments.

Ca isotopic compositions of dolomite, phosphorite and the oldest animal embryo fossils from the Neoproterozoic in Weng'an, South China

Deciphering the drastic changes of surface environment and the emergence of animals after the Neoproterozoic Snowball Earth event are important for understanding the changes of surface environment and its influence on the evolution of life through geologic time. Especially, the emergence of two types of Metazoan animals such as animal embryo fossils, cnidarians or sponges, and Ediacaran fauna in the late Neoproterozoic was one of the critical turning points in the biological evolution. Calcium is one of the essential elements for the growth of most animals. In this study, in order to evaluate the Ca cycles in the Neoproterozoic, we have measured Ca isotopic ratios ( 44Ca/ 42Ca and 43Ca/ 42Ca) for phosphorite, dolostone and phosphatic animal embryo fossils with a multiple collector, inductively coupled plasma-mass spectrometer (MC-ICP-MS). The resulting 44Ca/ 42Ca ratio defined by the relative deviation from the ratio of NIST SRM915a ( δ 44/42Ca NIST915) for phosphorite and dolostone ranges from 0.83 to 0.95‰, demonstrating that the fractionation between phosphorite/dolostone and seawater was very small. This evidence indicates that at the emergence of the Weng'an biota seawater was deficient in Ca probably due to mass deposition of phosphorite/dolostone and to the beginning of Ca-biomineralization. Three phosphatic animal embryo fossils have lower δ 44/42Ca values than the phosphorite and dolomite, implying that the precursor of the phosphatic embryo fossils was able to fractionate Ca isotopes through Ca-biomineralization, consistent with marine gastropods.

Snowball versus slushball Earth: Dynamic versus nondynamic sea ice?

Modeling studies of the Neoproterozoic snowball Earth offer two variations for snowball conditions, the original “hard” snowball Earth where the ocean is completely covered by sea ice, and an alternate slushball Earth or “soft” snowball, where there is an equatorial oasis of open water. We use the University of Victoria Earth System Climate Model to show that the soft snowball result is only possible when dynamics are excluded from the sea ice component of the model. Using a purely thermodynamic sea ice component the soft snowball condition is stable, whereas with a dynamic and thermodynamic sea ice component it is not. As the behavior of dynamic sea ice largely depends on wind stress, we compare simulations using two different wind fields: a zonally averaged present‐day wind field and a wind field derived by a general circulation model, the Fast Ocean Atmosphere Model, using Neoproterozoic conditions. Another consequence of using dynamic sea ice is that the sea ice does not become sufficiently thick to flow under its own weight when there is open water; this suggests that sea glacier dynamics are not important for snowball inception.

Deglaciating the snowball Earth: Sensitivity to surface albedo

The accepted values for both snow and ice albedo cover a large range, which is reflected in the values used in models studying the Neoproterozoic snowball Earth. The variation in these parameters can have a significant impact on modeling results and are often not explored within the modelling studies. Here we attempt to quantify the relative sensitivity of different surface albedos, using the University of Victoria's Earth System Climate Model, to deglaciating the snowball Earth. We investigate the sensitivity of ice, snow, and land albedo on the minimum CO2 greenhouse forcing required for deglaciating the Neoproterozoic snowball Earth. We find that the amount of CO2 forcing required for deglaciation can vary by nearly an order of magnitude within accepted albedo ranges.

The biotic response to Neoproterozoic snowball Earth

The commonly held notion among earth scientists that Neoproterozoic low latitude glaciation (ca. 720–585 Ma), sometimes referred to as snowball Earth, caused major extinctions and imparted important evolutionary consequences upon the biosphere is not supported by the microfossil record. In particular, silicified microfossils from pre- and syn-glacial units in the Death Valley region, California, reveal little change during the glacial interval; in fact, the syn-glacial microbiota is slightly more diverse and contains more putative autotrophic and heterotrophic eukaryotes than underlying strata. In Australia, pre- and post-glacial acritarch assemblages from shale reveal no change in diversity across the glacial interval. In modern glacial environments, productive and diverse modern microbial communities live within and upon sea and glacial ice and may provide an analogue for a more robust snowball Earth biosphere than previously considered.

Initial organic geochemical investigation on Late Neoproterozoic-Early Cambrian sediments in the Yangtze region, China*

Abstract Totally 19 samples of typical Upper Proterozoic-Lower Cambrian sedimentary rocks were collected and analyzed for an organic geochemical investigation. Almost all these rocks have high TOCs, super-maturities and similar biomarker distribution. As an exception, however, the Sinian Nantuo Tillite shows much lower TOCs and little phytane and pristane in comparison with those in other strata, which implies a very faint photosynthetic process, and a restricted euphotic zone and quite limited sunlight within the sedimentary water column during the Sinian glaciation age in the western Yangtze region providing an evidence for palaeo-oceanic environment of the Neoproterozoic Snowball Earth age.

The snow/ice instability as a mechanism for rapid climate change: A Neoproterozoic Snowball Earth model example

Paleoclimate data increasingly suggest the likelihood of abrupt transitions due to instabilities in the climate system. Several previous studies offer support for the snow/ice instability mechanism as an explanation for some of these changes. However, most of these studies have either employed simple models or not closely examined the details of the transition. Herein we revisit this issue using a general circulation model (GCM) simulation for the late Precambrian, a time when glaciers may have reached the equator. Our results indicate that for CO2 concentrations near present levels the snow/ice instability occurs in one model year. The width of open water associated with the onset of instability agrees with theoretical calculations of the critical length scale predicted for this behavior. These results strengthen support for the existence of this phenomenon as a mechanism for rapid climate change.

Variations of sulfur and carbon isotopes in seawater during the Doushantuo stage in late Neoproterozoic

Successive analyses of sulfur and carbon isotopic compositions of carbonates strata in the Doushantuo Formation in the Yangtze area were accomplished through a method of extracting trace sulfate from carbonates. Sulfur and carbon isotopic compositions of coeval seawater were estimated from the samples that show the least diagenetic alteration. A high-resolution age curve of sulfur isotopes in seawater sulfates was obtained in the Doushantuo stage, which reflects the trend of variation in seawater sulfur isotopes after the Neoproterozoic snowball Earth event. Similar characteristics of variation in carbon isotopes were observed in the coeval carbonates. A large positive δ34S excursion over +20‰ occurs in ancient seawater sulfates in the early Doushantuo stage. Simultaneously, the δ13 C values in ancient seawater carbonates exhibit a positive excursion up to 10‰ The maximum δ34S and δ13C values are +46.4‰ and +6.9‰, respectively. In the middle Doushantuo stage, the range of variation in δ34 S values of seawater is relatively narrow, but δ13C values are quite high. Then, δ34S values of seawater become oscillating, and the same occurs in δ13C values. Negative excursions in δ34 S and δ13 C values occur simultaneously at the end of the Doushantuo stage, and the minimum δ34S and δ13C values dropped down to −10.1‰ and –5.7‰, respectively. The characteristics of variations in the sulfur and carbon isotopes of ancient seawater imply strong changes in oceanic environment that became beneficial to inhabitation and propagation of organism. The organic productivity and burial rate of organic carbon once reached a quite high level during the Doushantuo stage. However, the state of environment became unstable after the global glaciation. The global climate and environment possibly were fluctuating and reiterating. The negative excursions in δ34S and δ13C values occurring at the end of the Doushantuo stage may represent a global event, which might be related to oxidation of deep seawater.

The snowball Earth hypothesis: testing the limits of global change

The gradual discovery that late Neoproterozoic ice sheets extended to sea level near the equator poses a palaeoenvironmental conundrum. Was the Earth's orbital obliquity > 60° (making the tropics colder than the poles) for 4.0 billion years following the lunar‐forming impact, or did climate cool globally for some reason to the point at which runaway ice‐albedo feedback created a `snowball' Earth? The high‐obliquity hypothesis does not account for major features of the Neoproterozoic glacial record such as the abrupt onsets and terminations of discrete glacial events, their close association with large (> 10‰) negative δ13C shifts in seawater proxies, the deposition of strange carbonate layers (`cap carbonates') globally during post‐glacial sea‐level rise, and the return of large sedimentary iron formations, after a 1.1 billion year hiatus, exclusively during glacial events. A snowball event, on the other hand, should begin and end abruptly, particularly at lower latitudes. It should last for millions of years, because outgassing must amass an intense greenhouse in order to overcome the ice albedo. A largely ice‐covered ocean should become anoxic and reduced iron should be widely transported in solution and precipitated as iron formation wherever oxygenic photosynthesis occurred, or upon deglaciation. The intense greenhouse ensures a transient post‐glacial regime of enhanced carbonate and silicate weathering, which should drive a flux of alkalinity that could quantitatively account for the world‐wide occurrence of cap carbonates. The resulting high rates of carbonate sedimentation, coupled with the kinetic isotope effect of transferring the CO2 burden to the ocean, should drive down the δ13C of seawater, as is observed. If cap carbonates are the `smoke' of a snowball Earth, what was the `gun'? In proposing the original Neoproterozoic snowball Earth hypothesis, Joe Kirschvink postulated that an unusual preponderance of land masses in the middle and low latitudes, consistent with palaeomagnetic evidence, set the stage for snowball events by raising the planetary albedo. Others had pointed out that silicate weathering would most likely be enhanced if many continents were in the tropics, resulting in lower atmospheric CO2 and a colder climate. Negative δ13C shifts of 10–20‰ precede glaciation in many regions, giving rise to speculation that the climate was destabilized by a growing dependency on greenhouse methane, stemming ultimately from the same unusual continental distribution. Given the existing palaeomagnetic, geochemical and geological evidence for late Neoproterozoic climatic shocks without parallel in the Phanerozoic, it seems inevitable that the history of life was impacted, perhaps profoundly so.

Neoproterozoic snowball Earth under scrutiny: Evidence from the Fiq glaciation of Oman

The Fiq Member of the Ghadir Manqil Formation, Huqf Supergroup, Oman, is composed of ∼1.5 km of glacial rainout diamictites, mass-flow deposits, turbiditic sandstones, and slope and shelf sandstones and siltstones, overlain by a classic <10-m-thick transgressive cap dolostone. Paleocurrents, facies distribution, and subsurface imagery show that the Fiq Member was deposited in a series of extensional half-grabens and grabens. Facies associations represent proximal glaciomarine, distal glaciomarine, nonglacial sediment gravity flow, and nonglacial shallow-marine siliciclastic depositional environments and processes. The member is divided into seven units that can be correlated across the rift basin. Although the facies mosaic is complex, cycles of relative sea-level change, most likely driven by glacio-eustasy, show that glacial advance and retreat during the glacial epoch was strongly pulsed. Only the final (youngest) diamictite is overlain by a well-developed cap carbonate. It is unlikely that there was any prolonged and substantial shutdown of the hydrologic cycle during deposition of the Fiq Member. On the basis of sedimentological and stratigraphic data, the Neoproterozoic glacials of Arabia were more like the familiar oscillatory glaciations of the Pleistocene than those required by the snowball Earth hypothesis.

An Early Snowball Earth?

In their article “A Neoproterozoic snowball Earth” (Reports, 28 Aug., p. [1342][1]), Paul F. Hoffman et al. report that global ice-house conditions existed during the Proterozoic, as inferred from negative carbon isotopes in carbonate rocks from Namibia. These conditions are said to have led to